The direct burning of biomass credits lot of CO2 into the atmosphere. Moreover, these biomasses are not used even as fodders, so they create environment related issues. Nevertheless, this huge biomass might be used for the production of second generation value added bio-fuels and bio-chemicals by an environment-friendly and economical process. Hence, the isolation of major bio-polymers is utmost important for advance synthesis of valuable products through chemical or biological reaction pathway.
In a future bio-economy, finding a suitable solvent for lignocellulosic biomass has become utmost important for generating renewable bio-chemicals and biofuels processing. Tremendous effort is currently being expended in the World, to find feasible pathways for the effective isolation of lipids, cellulose, hemicellulose and lignin from the agricultural residues. Conventional methods for biomass deconstruction into cellulose, hemicelluloses and lignin bio-products often require extreme conditions and expensive techniques (e.g. steam explosion, high temperatures, high pressure, addition of strong acids or bases or oxidizing chemicals) resulting in degradation and occurrence of undesired side reactions (e.g. the synthesis of furans). Moreover, new ionic liquid (ILs) technologies for large scale application still show limitations in terms of the recoverability and cost.
Mentioned herein below are some of the processes reported for the isolation of cellulose, hemicellulose and lignin along with their demerits.
In the literature reported on the isolation of cellulose from wood biomass, most of the processes use mineral acids (HCl, H2SO4, HNO3) or H2O2 or dioxane-mineral acids for isolation of cellulose from wood biomass or agricultural residues (Behera et al., Renew Sustain Energ Rev 36, 2014, 91). But, these processes required high temperature and a high concentrated solution of these acids for the isolation of major biopolymers.
Few reports are also available on the isolation of cellulose and hemicellulose from wood biomass by using aqueous solution of oxalic acid, maleic acid, etc (Vom Stein et al., Green Chem 13, 2011, 1772; Lee and Jeffries, Bioresour Technol 102, 2011, 5884). However, these organic acids extraction need high temperature (100 to 150° C.) and high pressure (upto 20 bar) for isolation of cellulose and hemicellulose.
Some of the work also reported isolation of cellulose and hemicellulose from wood biomass or agricultural residues in concentrated alkali medium at very low temperature upto −25° C. (Li et al., Ind Crop Prod 32, 2010, 551; Isogai, Cellulose 5, 1998, 309). However, the solvent attained this low temperature at the expense of high energy input. This process requires several attempts for effective isolation of cellulose and hemicellulose from the biomass. Moreover, the original crystallinity of the isolated cellulose and hemicellulose has been reduced.
Isolation of cellulose, hemicellulose and lignin using very costly ionic liquids (ILs) viz. 1-ethyl-3-methyl imidazolium acetate, 1-ethyl-3-methyl imidazolium chloride, 1-butyl-3-methyl imidazolium chloride, etc is also reported (Wang et al., CN103088692, 2013). The preparation of these solvents requires extensive chemical synthesis and purification steps. Further, the ILs are only effective in their purest forms, which adds cost to the procedure.
Some preliminary work on the separation of lipids and waxes together as crude extract was reported as pre-treatment process (Sasmal et al., Biomass Bioenerg, 45, 2012, 212). However, the systematic work on isolation, purification and characterization of lipids from spent aromatic crops biomass is not reported so far. The above methods for isolation of cellulose from plant biomass have serious drawbacks as they require hazardous reagents and are energetically wasteful. The most serious problem being the significant damage that occurs to the cellulose or hemicellulose or lignin during the extraction process. Therefore, an environmentally benign method for isolation of cellulose, hemicellulose and lignin without sacrificing the structure/properties of these biopolymers is the need of the hour. Though the extraction processes involving ILs have some advantages, yet they could not be further scaled-up due to its expensive nature. Therefore, the inventors of the present invention realized that there exists a dire need to provide some new model solvent systems to surmount the problems associated with using the IL solvents alone.
It is known that cellulose is a bio-polymer of glucose unit and hemicellulose is a bio-polymer of C6 and C5 sugars. Therefore, a number of high value chemicals can be derived from these carbohydrate polymers including hexitols, pentitols, levulinic acid, hydroxymethylfurfural, succinic acid, etc. Similarly lignin is viewed as an important bio-renewable source of aromatic compounds including vanillin, isovanillin, eugenol, isoeugenol, etc. Therefore, the development of an integrated bio-refinery is only possible if the isolation of these major biopolymers is done without sacrificing the hemicellulose or lignin. Most of the known procedures either target hemicellulose or lignin along with cellulose and during the isolation step one of the biopolymers (hemicellulose or lignin) is partially or completely lost or isolated in its distorted form.
In the hitherto known processes, there is requirement of corrosive solvents for carrying out the reaction at high operating reaction temperature, leading to corrosion of the metal reactors. Another category of processes needed very low operating temperature and also required repeated exercise for effective isolation of cellulose and hemicellulose from the wood biomass or agricultural residues. In yet another kind of processes the solvent (ILs) used are very expensive; therefore the processes are limited for laboratory studies only and cannot be scaled up further for commercialization. Overall the prior art processes are operable only with intensive input of energy. The present invention overcomes the said limitations by using solvent systems comprising ionic liquids for the isolation of lignin and other valuable biopolymers from spent biomass.